The fate of rainfall in a mallee and heath vegetated catchment in southern Western Australia

Abstract Mallee, a shrub‐eucalypt association, once covered large areas of the cereal growing land in Western Australia. The hydrologic consequences of land development have been more than a doubling of water yield and increased deep drainage beyond the plant‐root zone. The latter has led to large areas of soil salinization. The uncleared catchment studied was covered with mallee vegetation (65%) and a heath association (35%). Over 12 years of measurement, runoff from the catchment has averaged 0.025 mm per annum with a mean annual rainfall of 376 mm. Half of the runoff resulted from two major events totalling 0.15 mm. Since there was no evidence of groundwater accession in the catchment, the rainfall was balanced by evapotranspiration. Despite virtually no runoff from the whole catchment, considerable internal redistribution of surface water occurred, with localized overland flow in some areas as high as 7.7 mm from 30.9 mm of rainfall in one day. Rainfall penetrated rapidly under the mallee vegetation and the rate of penetration observed could not be achieved solely through the soil even if there was saturated flow. A typical stand of native mallee trees (Eucalyptus pileata and E. eremophila) in the catchment effectively redistributed 8% of the annual rain falling on the stand with 3% lost as interception and 5% going to stemflow. However, on an individual tree basis some 15% of the rain falling on the canopy was lost as intercepted water and 25% ran down the stem. The stemflow caused saturated conditions around the bole of the mallee and dye tracing showed that the water penetrated the soil via the annular pathways of the soil‐root interface. Roots of mallee trees were found at 28 m depth and it is postulated that the mallees are adapted to the semi‐arid environment by virtue of their ability to store water deep in the soil profile for use during the dry summer months.